Many computer peripherals such as CDROM and disc drives are connected in operation with a lightweight, portable computer such as a laptop. Such peripherals are battery powered and their batteries require periodic battery charging to sustain high performance. Such peripherals are typically compact and are not equipped with separate AC power converters or other charging platforms. Thus, techniques for delivering power to the peripherals for their sustained operation have included dedicated power signal lines within the interface cable that also performs the logic and communication functions. One such technique is described in the Universal Serial Bus Specification, Revision 1.0, available from In-System Design of Boise, Id., assignee of the present invention.
With increased capacity and higher speed peripherals such as disc drives, higher power is consumed. This is especially true for disc drives during start up (so-called spin up) of the spindle drive motor after a period of dormancy. There is also higher power consumption during the taxing seek cycles between adjacent tracks when the disc's read/write head's voice coil is repeatedly, rapidly started and stopped. The result of sustained operation of higher power peripherals connected to computers have demanded more power than could be provided by the computer through the interface while remaining within the bus specified power.